Digital satellite broadcast first begun in the United States in 1994 has entered into a practical stage for actual use after it has propagated to Japan, European nations, and the other countries in 1996. At the same time, there has been advancement in development of high-speed digital interfaces for receiving such digital signals via digital broadcasting, and transferring them isochronously (in real time) to individual receivers. In particular, a serial bus of the IEEE 1394 standard is expected to be the interface suitable for multimedia transmissions dominated primarily by moving video images, for the reason of such advantages as a high data rate and its ability to carry isochronous transmission and asynchronous transmission simultaneously.
Described hereinafter pertains to a transmitter of the prior art for receiving the above-mentioned broadcast and transmitting it to individual receivers via a serial bus, and a receiver of the prior art for receiving transmitted packet data.
The transmitter of the prior art analyzes a transport stream received in it, and detects a reference time stamp transmitted at approximately 100 msec intervals. The reference time stamp is a time reference derived from counting a system clock (27 Mhz) of the broadcasting station.
The transmitter comprises a voltage-controlled oscillator for generating a clock of 27 Mhz, a counter for counting the clock output by the voltage controlled oscillator, and a PLL circuit for latching a counted value of the counter at a timing of which the reference time stamp is detected and controlling an oscillation frequency of the voltage controlled oscillator according to a difference between the latched value and a value of the reference time stamp.
This PLL circuit reproduces a clock of 27 Mhz, which is synchronized with the system clock of the broadcasting station, and the time reference. The transmitter then adds the reproduced reference time stamp and a transmission time stamp as a header information to every packet data transferred via the IEEE-1394 serial bus. Then, the transmitter converts the transport stream received therein into a transport stream of the IEEE-1394, and transmits it to the receiver.
The receiver of the prior art receives the transport stream of the IEEE-1394 transmitted by the transmitter, and detects the transmission time stamp. The receiver restores the transport stream to the original state before the conversion with the detected transmission time stamp, and detects the reference time stamp.
A PLL circuit composed similarly in the receiver, as that of the transmitter, then, reproduces the system clock of 27 Mhz and the time reference.
If the receiver serves as an output device, it decodes the data with this reproduced system clock. On the other hand, if the receiver functions as a recording device, it produces a recording time stamp from the reproduced time reference, and records the time stamp together with the data in a recording medium.
However, both of the above-described data transmitter and the data recording device have a problem that they need to carry a large mass of hardware, as they require the PLL circuit for generating a clock in synchronization with the clock of the broadcasting station, when the data transmitter transmits the data to the data recording device.